1. Field of the Invention
The present invention relates to a microwave tube having a high frequency output section coupled to an output cavity.
2. Description of the Related Art
A large power klystron has been known as a microwave tube using the linear beam. The klystron is composed of: a klystron body including an electron gun for generating an electron beam, an input section for inputting high frequency power, a high frequency interacting section for amplifying high frequency power through the interaction of the electron beam with a high frequency electric field, a high frequency output section with a high frequency window for outputting the high frequency power that is amplified by the high frequency interacting section, and a collector section for collecting the electron beam that is no longer needed; and a magnetic field focusing device, mounted to and around the klystron body, for reducing the diameter of the electron beam to be a given diameter (Jpn. Pat. Appln. KOKAI Publication No. 11-149876, pages 2 to 3, FIGS. 1 and 2).
In some of this type of klystron, a plurality of high frequency output sections are coupled to the output cavity in order to cope with the power withstanding of the high frequency window or to meet client's requests.
If the coupling parts to the output cavity, the high frequency windows and the like, which are provided for one of those, for example, two high frequency output sections, are electrically and exactly the same as those for the other high frequency output section, the high frequency power output from one high frequency output section is exactly equal to that output from the other one. However, those high frequency output powers are minutely different from each other because of variations of the mechanical dimension of the coupling part to the output cavity and the high frequency window, variation of the relative permittivity of the dielectric member attached as the air-tight member to the high frequency window, and deformation of the wave guide. In the case where the matching of those high frequency output powers from the two high frequency output sections is lost, returning high frequency waves occur. This results in highering of VSWR (voltage standing wave ratio).
The difference between those two output powers is within 5% when the VSWR is low, in which case no problem arises. When the output power difference becomes a problem, a high frequency power mixer/divider 1 as shown in FIG. 7 is used. Generally, in the power mixer/divider 1, the high frequency powers output through two high frequency windows 2 are changed in traveling directions at corners 3, are mixed by a magic tee 4, and the mixed power is divided again into two high frequency powers at another magic tee 4, and those high frequency powers are changed in traveling directions at corners 5, and finally output to outside.
When the power mixer/divider 1 is used for the klystron, however, the external dimension of the klystron becomes large. Even when the power mixer/divider 1 is used, the two output powers could be exactly equal to each other if the electrical symmetry is secured. Actually, however, an output power difference inevitably occurs since the dimension accuracy variation of the magic tees 4 and other parts at the manufacturing stage is present.